1. Field of the Invention
This invention relates to a process for removing free aldehydes from compositions containing them.
2. Background Art
Aldehydes, and especially formaldehyde, are known irritants. Furthermore, some aldehydes, for example formaldehyde and acetaldehyde, are considered possible carcinogens. The latter are among the most important industrial chemicals, are widely used and reacted, but are also inadvertently released.
Formaldehyde, for example, is frequently used in the production of woodbase materials as a phenol-formaldehyde resin or a melamine-formaldehyde resin. Woodbase materials employing such resins as adhesives or binders regrettably release formaldehyde into the environment after production. Owing to its high volatility, formaldehyde is also frequently to be found in the offgas of a variety of industrial processes. For example, when textiles are treated with formaldehyde derivatives such as dimethyloldihydroxyethyleneurea (DMDHEU) or other methylol-functional compounds, they will release a not inconsiderable portion of the chemically-bound formaldehyde into the environment as free formaldehyde. As a further example, houses built with formaldehyde-releasing materials may release formaldehyde into the rooms through the masonry. One method of addressing this problem is presented in DE 19809479 A1, which describes a method for remediating formaldehyde-polluted interiors through the use of sheep""s wool.
Polymer dispersions are frequently prepared using formaldehydic compounds as fungicides, monomers or polymerization initiators. As a result, the ready-produced dispersion will contain major amounts of formaldehyde which can be released in later use. For example, it is known to use polymer dispersions as binders for consolidating and coating fibrous structures such as wovens, nonwovens and waddings comprised of textile fiber, textile yarns, or woodbase materials. The polymer dispersions are frequently copolymer dispersions of (meth)acrylate or vinyl ester copolymers which contain self-crosslinking comonomer units with N-methylol or N-methylol ether functions to improve strength. The copolymers customarily contain up to 10% by weight of units derived from N-methylol(meth)acrylamide (NMAA). A disadvantage of these binders is the release of formaldehyde due to hydrolytic cleavage of the N-methylol functionality.
Various strategies are proposed in the literature for reducing the level of free formaldehyde in polymer dispersions. These strategies generally utilize alternative, formaldehyde-free initiator systems, for example ascorbic acid. Such systems are capable of lowering the formaldehyde content by up to 80%, but have the disadvantage that they are very costly. The dispersions thus produced, however, can be produced and processed using conventional formulations and equipment.
EP-0778290 (U.S. Pat. No. 5,744,418) describes a process in which formaldehydic polymerizable initiator systems are replaced by aldehydes masked by bisulfite. When no HCHO-containing monomers are used, this leads to almost HCHO-free dispersions ( less than 3 ppm HCHO).
U.S. Pat. No. 5,585,438, DE-A 3328456 (AU 8431608), U.S. Pat. No. 5,071,902, EP-A 0488605 (U.S. Pat. No. 5,143,954) and U.S. Pat. No. 5,326,814 disclose the use of N-methylolacrylamide as a crosslinker with conventional initiator systems, but with further addition of low molecular weight components as formaldehyde traps or acceptors to reduce free formaldehyde. The compounds chosen for use as formaldehyde traps react quickly and very quantitatively with formaldehyde to form harmless derivatives. Urea compounds, acetoacetates, dicyandiamide, and imidazoles have found application here. However, these formaldehyde scavengers work only over a limited pH range, lack stability in storage, or may have to be added in large quantities. Formaldehyde reductions of above 90% are achieved through use of such scavengers. The dispersions thus produced can likewise be produced and processed using conventional formulations and equipment. However, the scavenger may be responsible for adverse effects, for example discoloration in the course of the subsequent crosslinking or else an increased toxicity of the compounds added.
The present invention provides a process for reducing free aldehyde in aldehyde-containing or aldehyde-generating compositions without causing adverse consequences such as for example unpleasant odors, increased toxicity, or discoloration. These and other objects are surprisingly achieved by addition of thioamine compounds.
The present invention accordingly provides a process for reducing free aldehydes in dispersions, suspensions, emulsions or solutions, which comprises including therein thioamines of the general formula I or II: 
where Y and Z are each a divalent organic radical of at least two carbon atoms and R1 is an organic alkyl or aryl radical of 1 to 15 carbon atoms or hydrogen, or mixtures thereof.
Preference is given to compounds of the formula III or IV or their salts or esters: 
where R1 is an organic alkyl radical of up to 5 carbon atoms, an aryl radical, or hydrogen, R2 is xe2x80x94(COO)axe2x80x94E or xe2x80x94(CONH)axe2x80x94E, where a is 0 or 1 and E is an organic radical of up to 10 carbon atoms or hydrogen, R3 is a monovalent organic radical of up to 10 carbon atoms or hydrogen, R4 and R5 represent organic radicals of up to 10 carbon atoms or hydrogen and may be bonded to each other, subject to the proviso that R4 and R5 together must contain at least one carbon atom, and n is 0 or 1.
Particular preference is given to compounds of the formula III or IV or their salts or esters where R1 is hydrogen or acetyl, R2 is xe2x80x94COOH or hydrogen, R3 is hydrogen, R4 is hydrogen or methyl, R5 is xe2x80x94COOH, xe2x80x94COOR, xe2x80x94CH2xe2x80x94COOR, xe2x80x94CH2xe2x80x94CN, optionally substituted aromatic, and n is 0 or 1.
These compounds include, for example but not by limitation, L-cysteine, D-cysteine, D,L-cysteine, N-acetylcysteine, cysteamine, homocysteine or their salts, esters or amides thereof and also their addition products (aminothioacetals or aminothioketals) with glyoxylic acid and pyruvic acid and their esters and amides, benzaldehyde, salicylaldehyde, vanillin, anisaldehyde, piperonal, acetophenone, cinnamaldehyde, citral, glucose, fructose, acetoacetate ester and acetylacetone. These naturally occurring amino acid derivatives are particularly safe toxicologically.
In the process according to the invention, free aldehyde present or being formed in dispersions, suspensions, emulsions or solutions is effectively bound by chemical reaction according to the general equation (1): 
Surprisingly, the equilibrium of the reaction is distinctly on the side of the bound aldehyde even in the presence of water, so that this reaction can be used for removing aldehydes in water-containing systems. In this context, the compounds of the general formula I have the advantage that they are capable of binding aldehydes very quickly by chemical reaction. However, some of the compounds of the general formula I have the disadvantage that they are readily oxidizable (cysteine for example).
Permanent reduction of free aldehyde, for example in polymer dispersions, is thus not always attainable through use of compounds of the formula I, for example cysteine, alone. However, permanent reduction of free aldehydes is attainable by using compounds of the general formula II alone or in admixture with compounds of the general formula I. This is because compounds of the general formula II, being the stable storage form of compounds of the formula I, cannot be oxidized and can react with the free aldehyde as per the general equation (2): 
In addition to the bound aldehyde, the latter reaction produces a compound which is itself an aldehyde or ketone. However, these can be selected so that they have no unwanted odor, and/or are nontoxic. Representative examples are pyruvic acid and acetoacetate ester. The latter compound is especially advantageous in that it in turn can act as an additional aldehyde scavenger.
The process of the invention is generally useful for reducing aldehydes of the formula Rxe2x80x94CHO, where R may be an organic alkyl or aryl radical of 1-20 carbon atoms or hydrogen. The process of the invention is more preferably used for reducing butyraldehyde, acetaldehyde, chloral and formaldehyde and most preferably for reducing formaldehyde.
The process of the invention is generally useful for reducing aldehydes in dispersions, suspensions, emulsions and solutions, preferably dispersions and suspensions, more preferably polymer dispersions and suspensions, and most preferably, polymer dispersions.
Free aldehyde is reduced according to the invention by using compounds of the general formula I or their salts, amides or esters, in an amount preferably between 0.002% by weight and 20% by weight, or compounds of the general formula II or their salts, amides or esters in an amount preferably between 0.002% by weight and 20% by weight, or mixtures thereof, each percentage by weight being based on the aldehydic dispersions, suspensions, emulsions or solutions for which aldehyde reduction is sought.
In a preferred embodiment, polymer dispersions are mixed with the thioamine compounds of the invention. The dispersions thus obtained are stable in that they comprise only very low levels of free aldehydes, for example formaldehyde, even after prolonged storage. The dispersions thus treated provide unimpaired crosslinking performance, nor are the compositions colored after crosslinking. An advantage is the simplicity of use of the inventive process, in that the thioamine compounds of the invention may simply be added as an aqueous solution to the ready-produced polymer dispersion.
The combination of amino acids with vinyl ester dispersions is disclosed in JP-A 1192612. In contrast to the process of the subject invention, the addition of amino acids is used to stabilize the resultant polymer dispersion in order that a sedimentation may be avoided, and requires the presence of an acetoacetate-functionalized polyvinyl alcohol as a very specific protective colloid. The dispersions used in the subject process according to the invention are, in contrast sufficiently stable even without addition of cysteine or acetoacetate-functionalized polyvinyl alcohols. Sedimentation stability was not observed to improve on using cysteine, but also did not have any adverse consequences for the sedimentation stability of the dispersions. JP-A 1192612 does not mention the formaldehyde content in the dispersion being lowered, although the use of acetoacetates as HCHO scavengers is common knowledge in the literature. The present invention provides formaldehyde reduction even without the use of acetoacetate-functionalized polyvinyl alcohols, by sole use of cysteine as an HCHO scavenger. Thus, in the present invention, it is desirable that the polymer dispersion be stable with respect to sedimentation prior to addition of the formaldehyde scavengers of the present invention. It is also preferable that the polymer dispersions be free of acetoacetate-functionalized polyvinyl alcohols.
The process according to the invention is particularly suitable for use in polymer dispersions or polymer suspensions containing one or more monomer units of vinyl esters of branched or unbranched carboxylic acids of 1 to 12 carbon atoms; esters of ethylenically unsaturated carboxylic acids, for example the esters of acrylic acid, methacrylic acid, maleic acid or fumaric acid with branched or unbranched alcohols of 1 to 12 carbon atoms; ethylenically unsaturated carboxylic acids; ethylenically unsaturated carboxamides; ethylenically unsaturated sulfonamides; styrenics; vinyl halides; alpha-olefins; or multiply ethylenically unsaturated compounds.
The vinyl ester copolymers or (meth)acrylic ester copolymers are preferably prepared by the emulsion polymerization process by known industrial processes. Useful dispersants include all emulsifiers and protective colloids customarily used in emulsion polymerization. The desired pH range for the polymerization, which is generally between 3 and 7, can be established in a known manner by means of acids, bases or customary buffer salts, such as alkali metal phosphates or alkali metal carbonates. Molecular weight reduction can be achieved in the polymerization by adding customary regulators.
Dispersions which have been admixed with thioalkylamines according to the process of the invention to reduce the level of free formaldehyde may be used to prepare dispersion powders. Dispersion powders are prepared by drying, preferably spray drying or freeze drying, most preferably spray drying, the dispersion. Conventional equipment is used, for example multi-component nozzles or disk sprayers dispensing into an optionally heated drying gas stream. The temperatures are generally not above 250xc2x0 C. The best temperature for the drying gas can be determined routinely; temperatures above 60xc2x0 C. have generally proven particularly useful.
The storability of powders having a low glass transition temperature (Tg) may be enhanced, and caking and blocking prevented, by adding antiblocking agents such as aluminum silicates, diatomaceous earth, or calcium carbonate, during the drying process. The dispersion may further include defoamers, for example silicone- or hydrocarbon-based defoamers, or atomizing aids, for example polyvinyl alcohols.
In preferred embodiments, the dispersion powders further contain up to 30% by weight, more preferably 1 to 15% by weight, based on base polymer weight, of a polyvinyl alcohol having a degree of hydrolysis of 85 to 94 mol %, and/or up to 10% by weight of vinyl alcohol copolymers containing 5 to 35% by weight of 1-methylvinyl alcohol units, and/or up to 30% by weight, more preferably 4 to 20% by weight, based on the total weight of polymeric constituents, of antiblocking agent and optionally up to 2% by weight, based on the base polymer, of defoamer.
Polymer dispersions or their powders incorporating the thioalkylamines of the invention may be used as compositions for coating aldehyde-emitting substrates, for example chipboard or materials of construction prepared therefrom. The coating may be effected directly or indirectly, for example by the dispersion being applied to a second substrate, and application or bonding to the formaldehyde-releasing substrate taking place before or after filming.
Aqueous solutions, for example wash solutions, can similarly incorporate the thioalkylamines of the invention in order to reduce aldehydes such as formaldehyde by, for example, washing textiles in these solutions, or by using such solutions to scrub aldehydic gases or gas mixtures, for example offgases.
In a further embodiment, emulsions as used for diverse applications, for example in the cosmetics industry, the pharmaceutical industry or the textile industry, may incorporate the thioalkylamines of the invention to reduce the level of undesirable aldehydes in these emulsions.
The examples below illustrate the invention, but should not be construed as limiting the scope of the invention.